Not gaining fast enough?

I'm starting to get a little worried about my baby's weight gain. She was born 9 lbs 11 oz, and she was gaining weight like CRAZY...at first. She turned 20 weeks yesterday, and she weighs a little over 15 lbs. I went to the lactation consultant, and they said she should weigh more at this point. They did say that she did not look like she was malnourished and her limbs and tummy look "plump". I am trying to feed her more frequently instead of waiting for signs of hunger (EBF). She has been sleeping through the night for three months now, so I do not plan on waking her up for a night feeding. Any suggestions? Should I start solids? Am I overthinking this? Thanks for any help or advice!